U.S. patent number 4,165,232 [Application Number 05/942,656] was granted by the patent office on 1979-08-21 for manufacture of ferromagnetic metal particles essentially consisting of iron.
This patent grant is currently assigned to BASF Aktiengesellschaft. Invention is credited to Rudolf Brodt, Christof Jaeckh, Eberhard Koester, Werner Loeser, Manfred Ohlinger, Werner Steck.
United States Patent |
4,165,232 |
Jaeckh , et al. |
August 21, 1979 |
Manufacture of ferromagnetic metal particles essentially consisting
of iron
Abstract
A process for the manufacture of acicular ferromagnetic metal
particles which essentially consist of iron and carry, on their
surface, from 0.02 to 0.6% by weight of boron in the form of a
borate, by reducing an acicular iron oxide with a gaseous reducing
agent at from 250.degree. to 500.degree. C., wherein a compound
selected from the group comprising boron oxyacids and their
inorganic salts is precipitated on the said iron oxide before
reduction.
Inventors: |
Jaeckh; Christof (Heidelberg,
DE), Steck; Werner (Mutterstadt, DE),
Brodt; Rudolf (Hirschberg, DE), Ohlinger; Manfred
(Frankenthal, DE), Loeser; Werner (Ludwigshafen,
DE), Koester; Eberhard (Frankenthal, DE) |
Assignee: |
BASF Aktiengesellschaft
(DE)
|
Family
ID: |
25478418 |
Appl.
No.: |
05/942,656 |
Filed: |
September 15, 1978 |
Current U.S.
Class: |
75/349;
252/62.55; 428/900; 148/105; 427/127 |
Current CPC
Class: |
H01F
1/061 (20130101); H01F 1/065 (20130101); Y10S
428/90 (20130101) |
Current International
Class: |
H01F
1/06 (20060101); H01F 1/032 (20060101); C22C
001/04 () |
Field of
Search: |
;75/123B,.5AA,.5BA
;148/105 ;252/62.55,62.56 ;427/127 ;428/900 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
|
|
|
2014500 |
|
Dec 1970 |
|
DE |
|
2212934 |
|
Mar 1972 |
|
DE |
|
1902270 |
|
Apr 1975 |
|
DE |
|
1907691 |
|
Apr 1975 |
|
DE |
|
47-16052 |
|
Dec 1972 |
|
JP |
|
Other References
Oppegard et al., "Magnetic Properties of Single-Domain Iron and
Iron-Cobalt Particles Prepared by Borohydride Reduction", Journal
of Applied Physics, Supplement to vol. 32, No. 3, Mar. 1961, pp.
1845-1855. .
Kneller, Ferromagnetismus, Springer-Verlag, Berlin (1962), pp.
437-439. .
Luborsky et al., "Crystallographic Orientation and Oxidation of
Submicron Whiskers of Iron, Iron-Cobalt and Cobalt", Journal of
Applied Physics, vol. 34, No. 9, Sep. 1963, pp. 2905-2909..
|
Primary Examiner: Rutledge; L. Dewayne
Assistant Examiner: Skiff; Peter K.
Attorney, Agent or Firm: Keil & Witherspoon
Claims
We claim:
1. A method of preparing acicular ferromagnetic metal particles
consisting essentially of iron and suitable for magnetic recording,
said particles being modified at the surface with 0.02 to 0.6% by
weight of boron as borate, by reducing a finely divided acicular
iron compound selected from the group consisting of iron oxide and
iron oxide hydrate with a gaseous reducing agent at a temperature
of from 250.degree. to 500.degree. C., wherein there are deposited
on said iron oxide or iron oxide hydrate, prior to reduction, a
substance selected from the group consisting of oxyacids of boron
and their inorganic salts in such an amount that 0.01 to 1% by
weight of boron is present.
2. A method of preparing acicular ferromagnetic metal particles
consisting essentially of iron and suitable for magnetic recording,
said particles being modified at the surface with 0.02 to 0.6% by
weight of boron as borate with 0.1 to 0.7% by weight of phosphorus
as phosphate and with 0.02 to 0.2% by weight of carbon, by reducing
a finely divided acicular iron compound selected from the group
consisting of iron oxide and iron oxide hydrate with a gaseous
reducing agent at a temperature of from 250.degree. to 500.degree.
C., wherein there are deposited on said iron oxide or iron oxide
hydrate, prior to reduction (a) a substance selected from the group
consisting of oxyacids of boron and their inorganic salts in such
an amount that 0.01 to 1% by weight of boron is present, (b) a
hydrolysis-resistant substance selected from the group consisting
of oxyacids of phosphorus, and their inorganic salts in such an
amount that 0.2 to 2% by weight of phosphorus is present, and (c) a
compound selected from the group consisting of aliphatic monobasic,
dibasic and tribasic carboxylic acids of from 1 to 6 carbon atoms
in such an amount that 0.1 to 1.2% by weight of carbon is present.
Description
The present invention relates to a process for the manufacture of
ferromagnetic metal particles essentially consisting of iron, which
are distinguished by a narrow particle size distribution coupled
with pronounced acicular shape, by reducing an acicular iron oxide
with a gaseous reducing agent.
Because of their high saturation magnetization and the high
coercive force achieved, ferromagnetic metal powders and thin metal
layers are of particular interest for the manufacture of magnetic
recording media. This is because they permit a substantial increase
in the energy product and the information density, which means,
inter alia, that using such recording media narrow signal widths
and better signal amplitudes than the conventional standard can be
achieved. Thin metal layers have the further advantage over
pigments that the ideal packing ratio of 1.0 is achievable since,
unlike the case of pigments, a binder is not needed. However, the
said metal layers are expensive to produce and in particular their
use as a tape recording medium presents problems because of the
mechanical characteristics of the tape. At the optimum layer
thickness of about 1 .mu.m or less, the surface of the layer must
be very smooth, because of head/tape contact, where the slightest
abrasion, or even dust alone, can be destructive.
It is true that when using metal powders as magnetic pigments the
mechanical properties of the recording medium can be varied within
wide limits by suitable selection of the binder system, but the
metal pigments have to conform to specific requirements in respect
of shape, size and dispersibility.
Since a high coercive force and high residual induction are
preconditions for magnetic pigments used in magnetic memory layers,
the corresponding metal pigments must exhibit magnetic
single-domain characteristics, and furthermore the existing
anisotropy, or the anisotropy additionally achievable by magnetic
orientation in the tape, should be relatively insensitive to
external factors, for example temperature or mechanical stress, ie.
the small particles should exhibit shape anisotropy, preferably by
being acicular, and should in general have a size of from 10.sup.2
to 10.sup.4 A.
The patent literature discloses numerous processes for the
manufacture of magnetic metal particles. For example, in the
process of U.S. Pat. No. 2,974,104 magnetic particles are deposited
by electroplating a liquid mercury cathode with iron from an
electrolyte solution. Thereafter, the particles must be separated
from the mercury by an expensive process.
The reduction of, for example, iron salts with hydrides (J. Appl.
Phys., 32 (1961), 184 S) and vacuum vaporization of metals,
followed by deposition as whiskers (J. Appl. Phys., 34 (1963),
2905) have also been disclosed, but are not relevant to industrial
practice. The manufacture of metal powders of the above type by
reducing finely divided acicular metal compounds, eg. oxides, with
hydrogen or some other gaseous reducing agent, has also been
disclosed. In order that the reduction shall take place at an
industrially useful rate, it must be carried out at above
350.degree. C. This however entails the difficulty that the metal
particles formed will sinter. As a result, the particle shape no
longer conforms to what is required from the point of view of the
magnetic properties. It has already been proposed to lower the
reduction temperature by applying silver or silver compounds to the
surface of finely divided iron oxide in order to catalyze the
reduction (German Laid-open Application DOS No. 2,014,500).
Modification of the iron oxide, to be reduced, with tin (German
published application DAS No. 1,907,691), with cobalt/nickel
(German published application DAS No. 2,212,934) and with
germanium, tin or aluminum (German published application DAS No.
1,902,270) have also been disclosed. However, the effect of the
said metals on the reduction of the acicular starting compounds is
in general to give much smaller needles than the starting material,
and furthermore the needles have a lower length/width ratio. The
result of this is that the end product exhibits a rather broad
particle size spectrum and, coupled therewith, a broad distribution
of shape anisotropy. However, the literature discloses that the
coercive force and residual induction of magnetic materials is very
dependent on the particle size when the latter is of the order of
magnitude of single-domain particles (Kneller, Ferromagnetismus,
Springer-Verlag 1962, page 437 et seq.). If to this are added the
factors resulting from the presence of a proportion of
superparamagnetic particles, which may be formed as fragments
during the above procedure, then such magnetic particles are highly
unsuitable for use in the manufacture of magnetic recording media,
for example because of their poor maximum output level at long
wavelengths. With such heterogeneous mixtures the magnetic field
strength required to reverse the magnetization of the particles
varies greatly and the distribution of the residual magnetization
as a function of the applied external field also results in a
rather flat residual induction curve.
It is an object of the present invention to provide a suitable
process for the manufacture of acicular ferromagnetic metal
particles which are distinguished by a narrow particle size
spectrum coupled with pronounced acicular shape of the particles
and which therefore exhibit high coercive force, a very steep
residual induction curve and little temperature dependence of
magnetic properties.
We have found that this object is achieved and that acicular
ferromagnetic metal particles essentially consisting of iron
conform to the above requirements if the surface of the metal
particles carries boron, in the form of a borate, in an amount of
from 0.02 to 0.6% by weight, based on the metal content of the
particles.
According to the invention, these metal particles, consisting
essentially of iron, are manufactured by reducing a finely divided
acicular iron oxide with a gaseous reducing agent at from
250.degree. to 500.degree. C., from 0.01 to 1% by weight of boron,
based on iron oxide, in the form of a boron oxyacid or an inorganic
salt thereof, being deposited on the iron oxide before
reduction.
All acicular iron oxides are suitable starting materials for the
manufacture of the metal particles of the invention, which
essentially contain iron. Preferably, these oxides are selected
from the group comprising alpha-FeOOH, gamma-FeOOH, mixtures of
these or iron oxides obtained from them by dehydration or heating,
Fe.sub.3 O.sub.4, gamma-Fe.sub.2 O.sub.3 and mixed crystals of
these, and alpha-Fe.sub.2 O.sub.3. These oxides may also be used
for the manufacture of ferromagnetic metal particles if they
contain other elements, provided that the latter do not interfere
with the acicular shape. Particularly advantageous ferromagnetic
particles contain iron and up to 25 atom per cent of cobalt.
For the purposes of the present invention it has proved
particularly advantageous to employ acicular goethite,
lepidocrocite or mixtures of these, with a mean particle length of
from 0.1 to 2 .mu.m, preferably from 0.2 to 1.2 .mu.m, a
length/width ratio of from 15:1 to 50:1, and a specific surface
area, S.sub.N.sbsb.2, of from 24 to 80 m.sup.2, preferably from 27
to 75 m.sup.2 /g. The dehydration products of the above hydrated
iron(III) oxides may be used similarly, the dehydration
advantageously being carried out in air at from 200.degree. to
600.degree. C.
A boron oxyacid or a salt thereof is now applied, according to the
process of the invention, to one of the above iron oxides. Examples
of suitable compounds are H.sub.3 BO.sub.3, HBO.sub.2, B.sub.2
O.sub.3, Na.sub.2 B.sub.4 O.sub.7.4H.sub.2 O, Na.sub.2 B.sub.4
O.sub.7.10H.sub.2 O, NaBO.sub.2, KBO.sub.2 and KB.sub.5
O.sub.8.4H.sub.2 O.
In developing the process of the invention it has proved
particularly advantageous to employ acicular goethite,
lepidocrocite or mixtures of these having a mean particle length of
from 0.1 to 2 .mu.m, preferably from 0.2 to 1.2 .mu.m, a
length/width ratio of from 15:1 to 50:1 and a specific surface
area, S.sub.N.sbsb.2, of from 24 to 80 m.sup.2, preferably from 27
to 75 m.sup.2 /g, which have not only been treated with one of the
above boron compounds but also with a phosphorus oxyacid or an
inorganic salt thereof, in an amount of from 0.1 to 0.7% by weight
of phosphorus, based on the iron oxide, and with an aliphatic
monobasic or polybasic carboxylic acid of 1 to 6 carbon atoms, in
an amount of from 0.1 to 1.2% by weight of carbon, based on iron
oxide. Reduction of such a material gives metal particles which in
addition to boron in the form of a borate also carry from 0.1 to
0.7% by weight of phosphorus in the form of a phosphate and carbon
in an amount of from 0.02 to 0.2% by weight. This additional
treatment may be carried out with phosphoric acid, a soluble salt
of orthophosphoric acid, eg. potassium orthophosphate, ammonium
orthophosphate, disodium orthophosphate, dilithium orthophosphate
or trisodium orthophosphate; a diphosphate, especially sodium
pyrophosphate, or a metaphosphate, eg. sodium metaphosphate. The
compounds may be used individually or as mixtures with one another.
The carboxylic acids may be saturated or unsaturated aliphatic
carboxylic acids of up to 6 carbon atoms and with up to 3 acid
radicals, with the hydrogens of the aliphatic chain being
unsubstituted or one or more of the hydrogens being substituted by
hydroxyl or amino. Particularly suitable acids are dicarboxylic
acids, hydroxydicarboxylic acids and hydroxytricarboxylic acids,
et. oxalic acid, tartaric acid and citric acid.
To finish the iron oxide, the latter may be suspended in water or a
water-soluble organic solvent, preferably a lower aliphatic
alcohol, or in a mixture of this organic solvent with water, but
preferably in water alone, by vigorous stirring. The appropriate
compounds are added to this suspension of oxide particles. To
ensure uniform dispersion, stirring is continued for some time,
advantageously for from 10 to 60 minutes, after adding the
compounds, and the mixture is then filtered. The finished oxide may
then be dried at up to 200.degree. C. in air or under reduced
pressure.
The amount of the substances present in the iron oxide suspension
is selected so that after the treatment the surface of the dried
product carries an amount of additive which is such that after
reduction the metal particles carry the amount according to the
invention. The concentration required to achieve this can, after
selection of the compounds to be used, easily be determined by a
few experiments and analytical determinations.
According to the process of the invention, the acicular oxide
treated as explained above is reduced to the metal by use of a
gaseous reducing agent. This may be carried out in a conventional
manner by passing a gaseous reducing agent, preferably hydrogen,
over the oxide at up to 500.degree. C., preferably at from
250.degree. to 450.degree. C. Iron needles having a length of from
0.1 to 0.8 .mu.m, with a length/width ratio of from 5:1 to 25:1,
may be mentioned as examples of products obtained according to the
process of the invention. The metal particles of the invention are
distinguished by particularly improved values of both the coercive
force and the residual induction, compared to the prior art.
The experiments which follow illustrate the invention.
The coercive force H.sub.c [kA/m], the specific remanence
M.sub.R/.rho. [nTm.sup.3 /g] and the saturation magnetization
M.sub.S .rho.[nTm.sup.3 /g] of the powder samples were measured in
a vibrating sample magnetometer at a field strength of 160 kA/m.
The coercive force H.sub.c was converted to the value corresponding
to the tap density 1.6, in accordance with the equation:
EXAMPLE 1
In six parallel batches A-F, 50 g samples of goethite having a
specific surface area, measured by the BET method, of 44.5 m.sup.2
/g, a particle length of 0.82 .mu.m and a length/width ratio of 35
are suspended in 750 ml of water, with vigorous stirring.
Batch A is filtered off without further treatment, as a Comparative
Experiment, and the filter cake is dried at 120.degree. C. under
reduced pressure. After reduction with 30 l/h of hydrogen at
350.degree. C. for 8 hours, an acicular iron powder is
obtained.
The follwing amounts of boric acid are added to batches B to F:
B: 0.25 g of H.sub.3 BO.sub.3,
C: 0.5 g of H.sub.3 BO.sub.3,
D: 1.0 g of H.sub.3 BO.sub.3,
E: 1.5 g of H.sub.3 BO.sub.3,
F: 2.0 g of H.sub.3 BO.sub.3,
in each case dissolved in 10 ml of water.
After stirring for a further 10 minutes, the solid is filtered off
and the filter cake is dried in air at 120.degree. C. Reduction of
this treated goethite at 350.degree. C. in a stream of hydrogen of
30 l per hour for a total of 8 hours gives an acicular iron powder.
The results are summarized in Table 1.
TABLE 1 ______________________________________ Content of B
Magnetic values at 160 kA/m Batch % H.sub.c(1.6) M.sub.m/.rho.
M.sub.r/92 M.sub.r /M.sub.m ______________________________________
A -- 54 146 75 0.51 B 0.03 67 120 67 0.56 C 0.05 68 119 66 0.55 D
0.11 70 124 70 0.56 E 0.16 71 140 79 0.56 F 0.17 73 135 74 0.55
______________________________________
EXAMPLE 2
In two parallel batches G and H, 50 g portions of alpha-FeOOH
having a specific surface area, measured by the BET method, of 42.4
m.sup.2 /g are suspended in 750 ml of water, with vigorous
stirring.
A mixture of 0.35 ml of 85% strength phosphoric acid and 0.5 g of
oxalic acid (C.sub.2 H.sub.2 O.sub.4.2H.sub.2 0) in 10 ml of water
is added to batch G. After stirring for a further 10 minutes, the
solid is filtered off and the filter cake is dried in air at
120.degree. C. Reduction of the treated goethite with 30 l/h of
hydrogen at 350.degree. C. for 8 hours gives an acicular iron
powder, which serves as a comparative sample.
A mixture of 0.35 ml of 85% strength phosphoric acid, 0.5 g of
oxalic acid (C.sub.2 H.sub.2 O.sub.4.2H.sub.2 O) and 0.5 g of boric
acid in 10 ml of water is added to batch H, which is then worked up
as described for batch G.
The analytical data and magnetic values of the iron powders
obtained are listed in Table 2.
TABLE 2 ______________________________________ Magnetic values at
160 kA/m Batch Content of H.sub.c(1.6) M.sub.m/92 M.sub.r/.rho.
M.sub.r /M.sub.m ______________________________________ G P = 0.4%
71 146 80 0.55 C = 0.08% P = 0.27% H C = 0.053% 74 136 76 0.56 B =
0.04% ______________________________________
EXAMPLE 3
5 kg of alpha-FeOOH having a specific surface area, measured by the
BET method, of 43.1 m.sup.2 /g are introduced, while stirring, into
a 60 l vessel containing 40 l of water. After dispersing for 10
minutes, 150 g of boric acid (H.sub.3 BO.sub.3), dissolved in 3 l
of water, are added.
After dispersion has been completed, the water is removed by
filtration and the finished hydrated iron(III) oxide is dried in
air at 140.degree. C. The dried pigment contains 0.36% of B.
The treated goethite is reduced to iron powder as described in
Example 1.
The iron pigment contains 0.48% of boron. The magnetic values,
measured at 160 kA/m, are
H.sub.c(1.6) =72.5
M.sub.m/.rho. =136
M.sub.r/.rho. =75
M.sub.r /M.sub.m =0.55
* * * * *